1. Field of the Invention
The present invention relates generally to the field of electromagnetic drivers, such as torque motors, solenoids and the like, and more particularly to improved apparatus for sensing the actual position of an armature in such an electromagnetic driver.
2. Description of the Prior Art
Torque motors and solenoids are perhaps the best known types of electromagnetic drivers.
Torque motors are commonly used in electrohydraulic servovalves to transduce an electrical input signal into a torque, which is used to control the flow of hydraulic fluid. Such torque motors generally include a permanent magnet, a polepiece, a coil, and an armature. While these components may be differently arranged, it is generally contemplated that the coil-induced flux interacts with the permanent magnet flux to produce a variable torque or force on the armature.
Another type of electromagnetic actuator is the solenoid, which generally includes a coil, an armature, and a magnetic circuit including an air gap. Current supplied to the coil induces a flux in the air gap, which in turn produces a force on the armature.
It is sometimes desired to determine the actual position of an armature in such electromagnetic drivers. For example, it may be desirable to verify the operation of a valve's electrical section prior to pressurization of its hydraulic section. Or, it may be desired to monitor armature position during normal operation to provide instrumentation data, for determining the fidelity of operation, or for fault monitoring in a redundant control system. In some applications, it may be desirable to determine armature position for use as a feedback signal to improve driver performance. In each case, it would be desirable to obtain such information concerning the actual position of the armature, without adversely affecting normal operation of the driver. Hence, it is important that such a position detector add negligible mass, inertia, or friction to the movable armature.
To date, several approaches have been proposed for sensing the position of an armature in a torque motor. These include: (1) attaching strain gages to the armature flexure support; (2) attaching the core of a linear variable differential transformer (LVDT) or a direct current differential transformer (DCDT) directly to the armature; (3) attaching one plate of a capacitance transducer to the armature; and (4) detecting reflected light from an incident light beam directed at the armature. Strain gages have not proved to be practical because of their frangibility. The second and third approaches add objectionable mass to the armature. The fourth approach does not add mass or friction, but requires additional space and weight for the light source and the beam detectors.
Specific examples of torque motors, used in electrohydraulic servovalves, are representatively shown in U.S. Pat. Nos. 2,625,136, 2,767,689, 3,023,782, 3,455,330, and 3,542,051. Use of torque motors in propellant control valves, is representatively shown in U.S. Pat. Nos. 3,373,769 and 3,884,267.